1. Field of the Invention
The present invention relates to a voltage detection device and an image heating device, which are used for an image forming apparatus, for example, a copier or a laser beam printer.
2. Description of the Related Art
A film-heating type image heating device for heat fixing of an image forming apparatus generally uses a heater in which a resistance heating element is formed on a substrate made of ceramics. With respect to the image heating device using the resistance heating element, in many cases, a heater of an image heating device used in an area where a commercial power supply voltage is a 100 V system (for example, 100 V to 127 V, hereinafter the same) has a different resistance value from a heater of an image heating device used in an area where the commercial power supply voltage is a 200 V system (for example, 200 V to 240 V, hereinafter the same). In order to realize a common universal image heating device which may be used in an area where both the commercial power supply voltage of the 100 V system and the commercial power supply voltage of the 200 V system are supplied, a method of changing the resistance value of the heater by using a switch, for example, a relay is proposed. For example, Japanese Patent Application Laid-Open No. H07-199702 or U.S. Pat. No. 5,229,577 describes an image heating device which includes first and second conductive paths in a longitudinal direction of the heater, in which the resistance value is changed between a first operating state where the first conductive path and the second conductive path are connected in series and a second operating state where the first conductive path and the second conductive path are connected in parallel.
In the method of switching between series connection and parallel connection of the two conductive paths as described in Japanese Patent Application Laid-Open No. H07-199702, a relay having a make contact (also called normally open contact) or a break contact (also called normally close contact) and a relay having a break-before-make contact (BBM contact) are used. Instead of the relay having the BBM contact, a relay having two make contacts or a relay having the make contact and the break contact may be used. In the proposed changing method described in U.S. Pat. No. 5,229,577, a relay having two BBM contacts is used. In this method, whether the power supply voltage is the 100 V system or the 200 V system is determined and the conductive paths of the heater are switched between the series connection and the parallel connection. Therefore, the resistance value of the heater may be changed without adjusting a heat generating region.
A known method for realizing a universally adaptable AC/DC converter used for the image forming apparatus is a method of changing a path for charging two electrolytic capacitors provided on a primary side of a transformer to switch between voltage doubler rectification and full-wave rectification. In this method, whether the power supply voltage is the 100 V system or the 200 V system is determined. When the power supply voltage is the 100 V system, the voltage doubler rectification is performed. When the power supply voltage is the 200 V system, the full-wave rectification is performed. Therefore, the voltage applied to the primary side of the transformer may be controlled to obtain a constant value as much as possible.
For example, Japanese Patent Application Laid-Open No. H05-030729 discloses a switching circuit for a rectifying circuit, in which an alternating-current (AC) input voltage is detected by a control circuit, and in response to a switching signal from the control circuit, the voltage doubler rectification of the 100 V system and the full-wave rectification of the 200 V system are switched by a switch element to obtain a constant direct-current (DC) output voltage. The feature of the switching circuit is that a gate element is used as the switch element and a pulse driving signal is generated as a gate element switching signal from the control circuit.
Meanwhile, power supplied to a fixing device which is a resistance load is proportional to a square value of an effective voltage value of the power supply. Therefore, it is desired to determine whether the conductive paths of the heater are to be connected in series or in parallel based on the effective voltage value or the square value thereof. In contrast, the voltage applied to the electrolytic capacitors of the AC/DC converter used for an image forming apparatus is a value close to a peak voltage (peak hold value) of the power supply. Therefore, it is desired to determine whether the power supply is set in the voltage doubler rectification state or the full-wave rectification state based on the peak voltage or quasi-peak voltage of the power supply.